SOP: Loss on Ignition

Basic Idea:

LOI is a standard technique used to assess the weight fraction of organic material within sediment. Estuarine sediment is primarily composed of water, root material, sediment organic matter, and inorganic (aka lithogenic) material. We first freeze dry the sediment in order to remove water. We then remove most root material while grinding the sediment. Root material actually contributes very little to the overall organic matter content of soil. The remaining two fractions are organic matter and inorganic material. When dried, ground sediment is burned at 550 deg C for 4 hours in a muffle furnace, effectively all organic material volatilizes, leaving only the inorganic fraction. By measuring the change in weight after burning, we can determine both the % organic matter, and % inorganic material.

Safety:

The muffle furnace will be 550 deg C…
- To prevent third-degree burns: don’t touch anything inside the furnace or anything that comes out of the furnace!!
- To prevent heat rash, minor burns, and singed hair: wear at least the face shield and a Kevlar glove on your dominant hand, as the gloves get hot change to another pair

Method:

Set up the balance area by laying out two clean metal spatulas on a kimwipe, a box of kimwipes, a squeeze bottle of tap water, compressed air, and the organized sediment samples.
Turn on the muffle furnace to 550 deg C about 45 minutes before burning sediment.
Wipe out inside of the small, blue container using kimwipes, then line the box with kimwipes.
Place the first 12 crucibles into the container using forceps, and carry the container closed to the lab.
Tare the balance.
Using the forceps, weigh the first crucible on the balance and record the value.
Open both side doors and place the jar/bottle of sediment open in front of the balance.
Using the clean, metal spatula weigh 1.0000 g of sediment +/- 0.0010 g into the crucible. The easiest way to do this is to have both hands inside the balance, sediment jar in the left hand and spatula in the right. Go slowly because cleaning off the balance takes more time. If you spill sediment inside the balance, remove the crucible using the forceps and place it on a kimwipe, lightly tapping to knock off loose sediment. Use the compressed air to spray away sediment inside the balance. Make sure to close the doors and retare the balance before weighing sediment out again. If you overfill sediment into the crucible, use the forceps to pick up the crucible and remove sediment using the spatula and dump it back into the sediment container.
Once you’ve weighed the correct amount, close the balance doors and record the weight.
Place the crucible very carefully back into the container using the forceps.
Repeat steps 4 – 9 on the next 11 samples.
Carry the crucibles and dirt in the closed container very carefully back to the 2nd floor lab and place them inside the desiccators.
Repeat steps 3-11 on the rest of the crucibles.
Once all of the crucibles are full of dirt, line each on up inside the 550 deg muffle furnace using the tongs. Make sure to wear safety gear and use the non-janky, long tongs to move each sample. **Be careful not to dip the tip of the tongs into the dirt. If you do, make a note of the sample number and clean off the tongs using a kimwipe.**
Turn on the drying oven to 60 deg C.
Wait 4 hr +/- 10 min.
Turn off the muffle furnace and lay a piece of aluminum foil below the furnace.
Wearing safety gear, open the muffle furnace. Use the forceps to transfer about 6 crucibles at a time to the aluminum foil. Close the furnace and open the drying oven. Place the crucibles inside. Repeat until all of the crucibles are in the drying oven.
Wait 1+ hr.
Transfer, using the forceps, to the desiccators. Make sure to place the correct number crucible in the correct place.
Wait 6+ hr (preferably overnight).
Place the open jar of desiccant inside the container.
Using the forceps, place the first 8 crucibles with ash inside the plastic container. Very carefully but quickly go back to the lab.
Quickly weigh each crucible and ash moving the crucibles with the forceps but keep the container closed as much as possible. Record the values. Make sure to tare the balance each time. Set each crucible aside.
Return to the 2nd floor lab and repeat steps 23-24 on the next 8 crucibles.

Cleaning crucibles:

After each use, crucibles must be cleaned to remove ash from the last LOI analysis and organic material from handling.

Turn the muffle furnace on to 550 deg C.
After the ashes and crucibles have been weighed, dump the ash in the trash.
Fill the 2, 500 ml beakers with a little alconox and warm water. Place the crucibles in the beakers.
One crucible at a time, use a pipe cleaner to scrub off material, rinse with warm water, and stack the crucibles in the plastic container.
Once all crucibles have been rinsed, line the crucibles in the muffle furnace.
Burn for 1 hr, then turn off the muffle furnace.
When reasonably cool, transfer using tongs to desiccators for storage. Make sure each crucible is placed in the correct location.

Ways to ruin the analysis:

Touching the crucibles with bare hands. Your fingers are covered in organic material (skin, sweat, etc.) and will affect the change in weight of the crucible and sediment. The only time you may touch the crucibles is after you have weighed the burned sediment (aka ash) and before you have fully cleaned the crucibles.
Mixing up the crucibles. Because sharpie will burn off at 550 deg C, the crucibles have a number etched into the bottoms. Be careful when reading these numbers so you don’t spill sediment, but make sure you have the correct crucible when recording values.
Recording values incorrectly. Take your time and check whether a number seems correct (e.g., the correct number of sig figs, weights similar to 1 g, no negative numbers).
Taking weight measurements with the doors on the balance open, the door to the lab open, or the window open. The balance is sensitive to changes in humidity and breezes so values will be incorrect if there is too much air flow in the lab. Additionally, the balance may never rest on a number if there’s a breeze.
Dropping the crucible, spilling sediment, etc. Obviously, the weight has now changed. Plus you may contaminate other samples.
Not burning the sediment for 4 hr +/- 10 min. This is very important for consistency between burns.
Leaving the ash outside of the drying oven or desiccators for too long. Once the organic matter has been burned out of the sediment, the ash will want to rehydrate quickly. Because the air is humid in Burt, this can happen quickly.

Based on Heiri et al. 2001.